Well it depends on how much yoru power company charges as to how much it costs to run really.
A friend of mine used one 24/7 for a few months, and foudn an average of a £15 increase in his power bill. Taking into account that his PC was heavily clocked with it, that was also the culprit for a percentage of the power consumption so I'd say closer to £10 per month 24/7.
If you don't use it 24 hours a day, it's a lot less.
I sort of think of a refrigerator. If you think about the cost to run one of those, you likely won't be that far off if you're not a 24/7 user.
But if you worked out your cost per whatever they measure you power in where you are, and refer to the power consumption average of the unit, you'll be able to work out what cost it actually is where you live.
Gray